Known brake boosters in the art are vacuum brake boosters using differential pressure between atmospheric pressure and vacuum and compressed air brake boosters using the pressure of compressed air. The vacuum brake booster applies large force to a brake by applying differential pressure between the atmospheric pressure and negative pressure generated in the vacuum pump or the intake manifold of the engine to a power piston in the brake booster.
A vacuum pump requires a specific apparatus, which increases manufacturing cost of a vehicle; therefore, a type of brake booster using the negative pressure generated in the intake manifold of the engine is commonly used.
However, the negative pressure generated in the intake manifold of the engine is not uniform, depending on the operational condition of the engine and the surroundings. Therefore, it was difficult to maintain stable negative pressure for a brake booster.